The impact of mathematics anxiety on the academic well-being of most in-school adolescents is quite frustrating. This development causes fear, apprehension and learned helplessness that can mar in-school adolescent's potentials. Therefore, in line with this context, this study through a pretest-posttest, control group quasi-experimental design investigated the effects of rational emotive behaviour therapy and emotional intelligence technique on the mathematics anxiety of in-school adolescents in Owerri Municipal Imo State Nigeria. Purposive sampling technique was used to select 60 participants from three randomly selected secondary schools in Owerri Municipal. The participants were randomly assigned to treatment and control groups. The two treatment groups were exposed to eight-weeks training in Rational Emotive Behaviour Therapy and Emotional Intelligence, while participants in the control group received no training. One validated instrument: Fennema-Sherman Mathematics Anxiety Scale (FSMAS) (alpha = 0.92) was used. Four hypotheses were tested at 0.05 level of significance. Data was analysed using Analysis of Covariance. There was significant main effect of treatment on the ability of in-school adolescents to manage mathematics anxiety (F (3, 56) = 26.98, p<0.05). Also, age was found to have significant main effect on the mathematics anxiety of in-school adolescents participants (F (2, 57) = 4.287, p<0.05). The results further indicated that there was no significant main effect of gender on the mathematics anxiety of in-school adolescents between male and female participants. Likewise, there was no significant interaction effect of treatment, age and gender on mathematics anxiety of in-school adolescent's participants. School counselling psychologists could adopt the two interventions to manage mathematics anxiety of in-school adolescents.